Gas Adsorption and Gas Storage in Organic-Rich Shale
Systems
Tongwei Zhang
Bureau of Economic Geology, The University of Texas at Austin
Summary: The effect of rock properties (organic matter content, type, maturity, and
clay minerals) on CH4 adsorption can be quantified with the heat of adsorption and the
standard entropy, which are determined from adsorption isotherms at different
temperatures. For clay-mineral rich rocks, the heat of adsorption (q) ranges from 9.4 to
16.6 kJ/mol. These values are considerably smaller than those for CH4 adsorption on
kerogen (21.9-28kJ/mol) and organic-rich shales (15.1-18.4kJ/mol). The standard
entropy (Δso) ranges from -64.8 to -79.5 J/mol/K for clay minerals, -68.1 to -111.3
J/mol/K for kerogen, and -76.0 to -84.6 J/mol/K for organic-rich shales. The affinity of
CH4 molecules for sorption on organic matter is stronger than for most common clay
minerals. Thus, it is expected that CH4 molecules may preferentially occupy surface sites
on organic matter. However, active sites on clay mineral surfaces are easily blocked by
water. As a consequence, organic-rich shales possess a larger CH4 sorption capacity
than clay-rich rocks lacking organic matter. The thermodynamic parameters obtained in
this study can be incorporated into model predictions of the maximum Langmuir
pressure and CH4 sorption capacity of shales under reservoir temperature and pressure
conditions.
Bio: Dr. Zhang is Research Scientist and Organic Geochemist with the Bureau of Economic Geology,
The University of Texas at Austin. He has joined UT Austin since 2007. He was a postdoctoral scholar
in chemistry at the California Institute of Technology (Caltech) (2001-2007). He holds a B.S. degree
in geology in Northwest University, China and a Ph.D. in isotope geochemistry in Chinese Academy of
Sciences. His research focuses on gas and organic geochemistry, isotope geochemistry, gas
generation kinetics and basin modeling, fluid transport processes in basins and reservoirs, gas
adsorption/desorption in shale, organic-inorganic interactions, and CO2 and H2S risk prediction. His
recent research interest is on shale gas geochemistry, gas adsorption and pore size distribution with
N2 adsorption. He got 2015 Wallace E. Pratt Memorial Award in the recognition of the significant
contribution to geology by the authorship of the 2013 AAPG Bulletin best paper titled “Organic
matter-hosted pore system, Marcellus Formation (Devonian), Pennsylvania”.